Editing Algol (section)

==System==
[[File:Algol AB movie imaged with the CHARA interferometer - labeled.gif|thumb|left|Algol Aa2 orbits Algol Aa1. This animation was assembled from 55 images of the [[CHARA array|CHARA interferometer]] in the near-infrared H-band, sorted according to orbital phase. Because some phases are poorly covered, Aa2 jumps at some points along its path.]] [[File:Algol Bahninterpolation.gif|thumb|left|alt=interpolation|Interpolation of the orbit of Aa2 around Aa1 with focus on Aa1.]]
Algol is a multiple-star system with three confirmed and two suspected stellar components.<ref>{{cite journal|arxiv=2005.13360|last1=Jetsu|first1=L.|title=Say Hello to Algol's New Companion Candidates|journal=The Astrophysical Journal |year=2021|volume=920 |issue=2 |page=137 |doi=10.3847/1538-4357/ac1351 |bibcode=2021ApJ...920..137J |s2cid=239882152 |doi-access=free }}</ref> From the point of view of the Earth, Algol&nbsp;Aa1 and Algol&nbsp;Aa2 form an [[eclipsing binary]] because their [[orbital plane (astronomy)|orbital plane]] contains the  [[line-of-sight propagation|line of sight]] to the Earth. The eclipsing binary pair is separated by only 0.062&nbsp;[[astronomical unit]]s (au) from each other, whereas the third star in the system (Algol&nbsp;Ab) is at an average distance of 2.69&nbsp;au from the pair, and the mutual [[orbital period]] of the trio is 681&nbsp;Earth days. The total mass of the system is about 5.8 solar masses, and the mass ratios of Aa1, Aa2, and Ab are about 4.5 to 1 to 2.

The three components of the bright triple star used to be, and still sometimes are, referred to as β Per A, B, and C.  The [[Washington Double Star Catalog]] lists them as Aa1, Aa2, and Ab, with two very faint stars B and C about one [[arcmin]] distant.  A further five faint stars are also listed as companions.<ref name=wds>{{cite journal|bibcode=2001AJ....122.3466M|title=The 2001 US Naval Observatory Double Star CD-ROM. I. The Washington Double Star Catalog|journal=The Astronomical Journal|volume=122|issue=6|pages=3466–3471|last1=Mason|first1=Brian D.|last2=Wycoff|first2=Gary L.|last3=Hartkopf|first3=William I.|last4=Douglass|first4=Geoffrey G.|last5=Worley|first5=Charles E.|year=2001|doi=10.1086/323920|doi-access=free}}</ref>

The close pair consists of a B8 [[main sequence]] star and a much less massive K0 [[subgiant]], which is highly distorted by the more massive star.  These two orbit every 2.9 days and undergo the eclipses that cause Algol to vary in brightness.  The third star orbits these two every 680 days and is an A or F-type main sequence star.  It has been classified as an [[Am star]], but this is now considered doubtful.<ref name=frank2022>{{cite journal |bibcode=2022JAVSO..50..123F |title=Spectral Classification of Algol C |last1=Frank |first1=M. G. |last2=Whelan |first2=D. G. |last3=Junginger |first3=J. C. |journal=Journal of the American Association of Variable Star Observers (Jaavso) |year=2022 |volume=50 |issue=1 |page=123 |arxiv=2205.06229 }}</ref><ref name=renson2009>{{cite journal
 | last1=Renson | first1=P. | last2=Manfroid | first2=J.
 | title=Catalogue of Ap, HgMn and Am stars
 | journal=Astronomy and Astrophysics
 | volume=498 | issue=3 | pages=961–966 | date=May 2009
 | doi=10.1051/0004-6361/200810788 | bibcode=2009A&A...498..961R | doi-access=free }}</ref>

Studies of Algol led to the [[Algol paradox]] in the theory of [[stellar evolution]]: although components of a binary star form at the same time, and massive stars evolve much faster than the less massive stars, the more massive component Algol&nbsp;Aa1 is still in the [[main sequence]], but the less massive Algol&nbsp;Aa2 is a [[subgiant star]] at a later evolutionary stage. The paradox can be solved by [[mass transfer]]: when the more massive star became a subgiant, it filled its [[Roche lobe]], and most of the mass was transferred to the other star, which is still in the main sequence. In some binaries similar to Algol, a gas flow can be seen.<ref>{{cite journal | first=Izold | last=Pustylnik | title=On Accretion Component of the Flare Activity in Algol | journal=Baltic Astronomy | date=1995 | volume=4 | issue=1–2 | pages=64–78 | bibcode=1995BaltA...4...64P | doi=10.1515/astro-1995-0106 | doi-access=free }}</ref> The gas flow between the primary and secondary stars in Algol has been imaged using Doppler [[Tomography]].<ref>{{Cite journal|last1=Richards|first1=Mercedes T.|last2=Albright|first2=Geary E.|last3=Bowles|first3=Larissa M.|date=January 1995|title=Doppler tomography of the gas stream in short-period Algol binaries|journal=The Astrophysical Journal|language=en|volume=438|pages=L103|doi=10.1086/187726|bibcode=1995ApJ...438L.103R|issn=0004-637X|doi-access=free}}</ref><ref>{{Cite journal|last1=Richards|first1=Mercedes T.|last2=Agafonov|first2=Michail I.|last3=Sharova|first3=Olga I.|date=2012-11-20|title=NEW EVIDENCE OF MAGNETIC INTERACTIONS BETWEEN STARS FROM THREE-DIMENSIONAL DOPPLER TOMOGRAPHY OF ALGOL BINARIES: β PER AND RS VUL|journal=The Astrophysical Journal|volume=760|issue=1|pages=8|doi=10.1088/0004-637X/760/1/8|arxiv=1210.0081|bibcode=2012ApJ...760....8R|s2cid=118319759|issn=0004-637X}}</ref>

This system also exhibits [[x-ray]] and [[radio wave]]<ref>{{Cite journal|last1=Wade|first1=C. M.|last2=Hjellming|first2=R. M.|date=February 4, 1972|title=Radio Stars Beta Persei and Beta Lyrae|journal=Nature|language=en|volume=235|issue=5336|pages=270–271|doi=10.1038/235270a0|bibcode=1972Natur.235..270W|s2cid=4222515|issn=0028-0836}}</ref> flares. The x-ray flares are thought to be caused by the magnetic fields of the A and B components interacting with the mass transfer.<ref>{{cite journal | author=M.J. Sarna | author2=S.K. Yerli | author3=A.G. Muslimov | title=Magnetic Activity and Evolution of Algol-type Stars - II | journal=[[Monthly Notices of the Royal Astronomical Society]] | date=1998 | volume=297 | issue=3 | pages=760–68 | bibcode=1998MNRAS.297..760S | doi=10.1046/j.1365-8711.1998.01539.x | doi-access=free }}</ref> The radio-wave flares might be created by magnetic cycles similar to those of [[sunspot]]s, but because the magnetic fields of these stars are up to ten times stronger than the field of the [[Sun]], these radio flares are more powerful and more persistent.<ref>{{cite web | last = Blue | first = Charles E. | date = 3 June 2002 | url = http://www.nrao.edu/pr/2002/algol/ | title = Binary Stars "Flare" With Predictable Cycles, Analysis of Radio Observations Reveals | publisher = National Radio Astronomy Observatory  | access-date = 31 July 2006 | archive-url= https://web.archive.org/web/20060702231546/http://www.nrao.edu/pr/2002/algol/| archive-date= 2 July 2006 | url-status= live}}</ref><ref>{{Cite journal|last1=Richards|first1=Mercedes T.|last2=Waltman|first2=Elizabeth B.|last3=Ghigo|first3=Frank D.|last4=Richards|first4=Donald St. P.|date=August 2003|title=Statistical Analysis of 5 Year Continuous Radio Flare Data from β Persei, V711 Tauri, δ Librae, and UX Arietis|journal=The Astrophysical Journal Supplement Series|language=en|volume=147|issue=2|pages=337–361|doi=10.1086/375835|bibcode=2003ApJS..147..337R|issn=0067-0049|doi-access=free}}</ref> The secondary component was identified as the radio emitting source in Algol using [[Very-long-baseline interferometry]] by Lestrade and co-authors.<ref name=lestrade1993>{{Cite journal|last1=Lestrade|first1=Jean-Francois|last2=Phillips|first2=Robert B.|last3=Hodges|first3=Mark W.|last4=Preston|first4=Robert A.|date=June 1993|title=VLBI astrometric identification of the radio emitting region in Algol and determination of the orientation of the close binary|journal=The Astrophysical Journal|language=en|volume=410|pages=808|doi=10.1086/172798|issn=0004-637X|bibcode=1993ApJ...410..808L}}</ref>

Magnetic activity cycles in the chromospherically active secondary component induce changes in its radius of gyration that have been linked to recurrent orbital period variations on the order of {{sfrac|ΔP|P}}&nbsp;≈&nbsp;{{val|e=−5}} via the [[Applegate mechanism]].<ref name="Applegate 1992">{{cite journal |last1=Applegate |first1=James H. |title=A mechanism for orbital period modulation in close binaries|journal=Astrophysical Journal, Part 1 |date=1992 |volume=385 |pages=621–629|bibcode=1992ApJ...385..621A|doi = 10.1086/170967 |doi-access=free }}</ref> [[Mass transfer#Astrophysics|Mass transfer]] between the components is small in the Algol system<ref>{{cite thesis |last=Wecht|first=Kristen |arxiv=astro-ph/0611855 |title=Determination of Mass Loss and Mass Transfer Rates of Algol (Beta Persei) from the Analysis of Absorption Lines in the UV Spectra Obtained by the IUE Satellite |date=2006 |bibcode=2006PhDT........10W }}</ref> but could be a significant source of period change in other [[Algol variable|Algol-type binaries]].
[[File:1e9m comparison Gamma Orionis, Algol B, the Sun, and smaller - antialiased no transparency.png|left|thumb|Size comparison between the [[Sun]] (bottom middle), Algol Aa2 (right) and the [[blue giant]] [[Bellatrix]] (left).]]
The distance to Algol has been measured using very-long baseline [[interferometry]], giving a value of 94&nbsp;[[light-year]]s.<ref name=xu/> About 7.3&nbsp;million years ago it passed within 9.8&nbsp;light-years of the [[Solar System]]<ref>{{cite conference
 | display-authors=4| author=Garcia-Sanchez, J.
 | author2=Preston, R. A.
 | author3=Jones, D. L.
 | author4=Lestrade, J.-F.
 | author5=Weissman, P. R.
 | author6=Latham, D. W.
 | title =A Search for Stars Passing Close to the Sun
 | book-title =The First Results of Hipparcos and Tycho
 | publisher =IAU | date =25 August 1997 | location =Kyoto, Japan
 | bibcode =1997IAUJD..14E..51G
 }}<!--| access-date = 1 June 2007--></ref> and its [[apparent magnitude]] was about −2.5, which is considerably brighter than the star [[Sirius]] is today. Because the total mass of the Algol system is about 5.8&nbsp;solar masses, at the closest approach this might have given enough [[gravity]] to [[Perturbation (astronomy)|perturb]] the [[Oort cloud]] of the Solar System somewhat and hence increase the number of [[comet]]s entering the inner Solar System. However, the actual increase in net cometary collisions is thought to have been quite small.<ref>{{cite journal | author = J. García-Sánchez | author2 = R.A. Preston | author3 = D.L. Jones | author4 = P.R. Weissman | title=Stellar Encounters with the Oort Cloud Based on Hipparcos Data | journal=The Astronomical Journal | date=1999 | volume=117 | issue = 2 | pages=1042–55 | doi=10.1086/300723 | bibcode=1999AJ....117.1042G| s2cid = 122929693 | doi-access=free }}</ref>